iio: adc: rzg2l_adc: Use adc-helpers

iio: adc: rzg2l_adc: Use adc-helpers

The new devm_iio_adc_device_alloc_chaninfo_se() -helper is intended to
help drivers avoid open-coding the for_each_node -loop for getting the
channel IDs. The helper provides standard way to detect the ADC channel
nodes (by the node name), and a standard way to convert the "reg"
-properties to channel identification numbers, used in the struct
iio_chan_spec. Furthermore, the helper can optionally check the found
channel IDs are smaller than given maximum. This is useful for callers
which later use the IDs for example for indexing a channel data array.

The original driver treated all found child nodes as channel nodes. The
new helper requires channel nodes to be named channel[@N]. This should
help avoid problems with devices which may contain also other but ADC
child nodes. Quick grep from arch/* with the rzg2l_adc's compatible
string didn't reveal any in-tree .dts with channel nodes named
otherwise. Also, same grep shows all the .dts seem to have channel IDs
between 0..num of channels.

Use the new helper.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://patch.msgid.link/e84ccff01070eaee3dd1ae2d7e109f963168f586.1742560649.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>